The present invention relates generally to incinerator systems for the abatement of process emissions and, more particularly, to a valve for the control of fluid flow to and from a regenerative incinerator.
Process emissions often contain combustible contaminants that, if released to the atmosphere, have the potential of polluting the environment. More particularly, noxious fumes, waste gas or process emissions generally contain contaminants in the form of volatile organic combustibles (VOCs).
However, the amount of combustible material contained in these emissions is generally below the concentration required to ignite or propagate a flame at ambient temperature. Accordingly, incinerators are employed to treat the process emissions by increasing the temperature of such process emissions to a temperature above the ignition temperature of the contaminants therein so as to oxidize the contaminants.
For increased efficiency, a regenerative incinerator may be employed, which has two or more regenerative beds, connected to a common combustion chamber. Each regenerative bed is filled with a solid packing, such as a multiplicity of heat retaining ceramic rings. In the case of a two-bed regenerative incinerator, gas flow is such that hot gas from the combustion chamber heats one incinerator bed while the other regenerative bed is used to preheat incoming process emissions. Gas flow through the regenerative beds is periodically alternated so that the one regenerative bed then preheats incoming process emissions, while the other regenerative bed is heated by hot gas from the combustion chambers. Regenerative preheating increases the overall efficiency of the incinerator by minimizing the amount of fuel required to raise the process emissions to ignition temperature.
It will be appreciated that a suitable valving arrangement is required to alternate the gas flow and thus the functions of the two (or more) regenerative beds. It will further be appreciated that demanding requirements are placed on flow control valves in a regenerative incinerator application. The valves must be relatively large to handle a high volume of gas flow, and further must operate over a typical incinerator temperature range of 50.degree. F. to 1,000.degree. F. (10.degree. C. to 538.degree. C.).
By way of more particular example, such regenerative incinerators are disclosed in Houston U.S. Pat. No. 3,870,474, entitled "Regenerative Incinerator Systems for Waste Gases"; Houston U.S. Pat. No. 5,000,422, entitled "Incinerator Valve"; and Houston U.S. patent application Ser. No. 08/067,665, filed May 26, 1993, entitled "Two Bed Thermal Regenerative Fume Incinerator", the entire disclosures of which are hereby expressly incorporated by reference.
The above-incorporated Houston U.S. Pat. No. 3,870,474 discloses a three-bed regenerator system employing three-way valves. The system is such that each bed, at different times, is a feed bed, a purge bed and an exhaust bed. The above-incorporated Houston U.S. Pat. No. 5,000,422 discloses a three-bed regenerative incinerator system employing simple butterfly valves which are either open or closed. The above-incorporated Houston application Ser. No. 08/067,665 discloses a two-bed regenerative incinerator which employs a four-way valve to reverse the flow through the two beds.
A problem which materially affects the effectiveness of such regenerative incinerators is leakage of untreated process emissions past the relatively large flow control valves required for control of fluid to and from the regenerative beds. For example, the two-bed regenerator disclosed in the above-incorporated application Ser. No. 08/067,665 has a single four-way valve to alternately control the flow to and from the two regenerative beds. The four-way valve has two interior spaces defined on either side of a two-position butterfly valve element. When the butterfly valve element is in either of its two positions there is untreated fluid in one interior space within the valve and treated fluid in the other interior space within the valve. Leakage of untreated fluid through the valve into the interior space with treated fluid seriously compromises the effectiveness of the incinerator system, and decreases purity of the incinerator exhaust emission.
Another problem is that, during the relatively short interval when the butterfly valve element is being operated and moving between the two valve positions, an inlet conduit containing untreated gas may be directly connected to an outlet conduit which should contain only treated gas. This can also seriously decrease the purity of the incinerator exhaust emission.
In the above-incorporated Houston U.S. Pat. No. 5,000,422, as well as in Houston U.S. Pat. No. 5,217,041, it is disclosed that a simple two-way on/off butterfly valve can be made leakproof by an annular plenum on the rim of the disk or flapper within the valve, and either pressurizing or purging the annular plenum. U.S. Pat. Nos. 5,000,422 and 5,217,041 disclose specific structures to define the plenum and to provide fluid communication with the plenum.
As another example, block and bleed valve arrangements have two main valves with a bleed valve in between. This has been used to eliminate the effect of valve leakage. However, with a multiport valve, such an arrangement is impractical.
Yet another example of a known valve arrangement which has better leakage characteristics is a ball valve with associated venting. However, for pipes over four inches in diameter, ball valves typically are not large enough to adequately function as incinerator control valves, or cannot be feasibly constructed of materials able to operate over a typical incinerator temperature range of 50.degree. F. to 1,000.degree. F. (10.degree. C. to 538.degree. C.), or both.